Abstract. -- Caddo Lake, located along the boundary of northeastern
Texas and northwestern Louisiana, contains extensive areas of relatively
undisturbed forested wetlands and has been designated a wetland of
international importance under the Ramsar Convention. Vegetation and
water levels from 30 plots were sampled. Aerial photographs of the 3000
ha Caddo Lake State Park and Wildlife Management Area were used to map
the park into 119 survey areas, each representing a distinct patch of
vegetation. Both data sets were separately subject to Two Way Indicator
Species Analysis (TWINSPAN) classification and Detrended Correspondence
Analysis (DCA) ordination. Water levels were strongly correlated with
the first axis of a DCA ordination of the 30 plots which arranged plots
along a flooding gradient from mesic natural levees and terraces to
cypress swamps. These results were corroborated by analysis of data from
the 119 survey areas and were used to identify six community types: Rich
Mesic Slopes, Mesic Bottomland Ridges, Bottomland Oak Flats,
Cypress-Water elm Swamps, Closed-Canopy Cypress Swamps and Deep water
Cypress Swamps. These communities are comparable to other wetland
communities described for the region.

**********

Forested wetlands in the southern USA have suffered extensive
losses since the time of European settlement (Mitsch & Gosselink
1993). Most remaining areas exist as isolated fragments of less than 100
ha (Gosselink et al. 1990). Between 1940 and 1980, the national rate of
loss of forested wetlands in the USA was 2.8 million ha per year with
most of the loss occurring in the South (Abernathy & Turner 1987).
In eastern Texas, only an estimated 37% of the presettlement riparian forests still existed in 1980 (Frye 1987). Although much of the loss is
due to conversion to agriculture, water resource projects are also a
threat; over 240,000 ha of Texas bottomland forest were destroyed by
impoundments such as Lake of the Pines, Toledo Bend Reservoir and Sam
Rayburn Reservoir (Frye 1987).

Caddo Lake, located in northeastern Texas and northwestern
Louisiana, is associated with 3000 ha of contiguous forested wetlands on
public lands (Taylor et al. 1996) and additional wetlands on private
lands. It provides an intact forested wetland landscape on a scale
seldom seen today in the South. Consequently, Caddo Lake is recognized
as a wetland habitat of international importance by the Ramsar
Convention, an international convention named after Ramsar, Iran, the
place of its adoption (Navid 1989; Davis 1994). Caddo lake, Catahoula
Lake in Louisiana, and the Okefenokee Swamp in Georgia are the only
three Ramsar wetlands in the southern USA excluding peninsular Florida
(US Fish and Wildlife Service 1996).

There have been many hydrologic, geomorphic and vegetation studies
of bottomlands and swamps in the South including Chambless & Nixon
(1975), Thompson (1980), Bedinger (1981), Mathies et al (1983), Miller
(1990), Nixon et al. (1990), Shankman & Drake (1990), Shankman
(1991), Barrett (1995) and Crouch & Golden (1997). However, in spite
of Caddo Lake's importance as a major wetland landscape, only
limited non-published information about its wetland vegetation (Hine
& Nixon 1992; Sheffield 1995; MacRoberts 1979) is available. This
paper provides a quantitative description and gradient analysis of the
wetland plant communities of Caddo Lake, focusing on the public lands of
the Caddo Lake State Park and Wildlife Management Area.

STUDY AREA

Caddo Lake lies between Mooringsport, Louisiana, and Karnack, Texas at approximately 32[degrees] 42' 38" N, 94[degrees] 8'
25" W along Big Cypress Bayou, a tributary of the Red River. It is
in the Pineywoods vegetation area of Hatch et al. (1990), and the
Southeastern Mixed Forest Province of Bailey et al. (1994). The climate
is humid-subtropical with a mean annual low temperature of 11[degrees]C,
an average high temperature of 22[degrees]C, an annual mean of 116.8 cm
of rainfall, and negligible snowfall (Larkin & Bomar 1983).

Historically called Ferry Lake, Caddo Lake is a drowned floodplain,
one of several lakes that formed around the year 1800 as a result of a
120 km-long series of log jams (the Great Raft) that blocked the main
channel of the Red river (Barrett 1995). The Raft forced water into side
channels of the Red River and caused water to back up into tributaries,
forming Caddo Lake and the now extinct Sodo Lake. Steam boats entered
Caddo Lake via side channels of the Red River, contributing to the
commerce of the time (Dahmer 1988). Removal of the Great Raft in 1873
drained Sodo Lake and caused Caddo Lake to slowly recede until a dam
built in 1914 near Mooringsport, Louisiana, stabilized the lake and
preserved the cypress and bottomland wetlands of the formerly natural
lake (Dahmer 1988; Barrett 1995). The extent of the alteration of the
original hydrologic regime and consequent influence on vegetation is
unknown, although sedimentation rates were higher prior to the break up
of the Raft and dam construction as the result of inflow of
sediment-rich water from the Red River (Barrett 1995).

Caddo Lake, including its wetlands, covers roughly 10,7200 ha at
capacity of 51.36 m above mean sea level, but there is considerable
seasonal variation in water levels and in the area actually flooded
(USGS 1:2400 Topographic map; A.I.D. Associates 1993; US Army Corps of
Engineers 1994b). The eastern portion of Caddo Lake, much of which lies
in Louisiana, is the deepest and consists largely of open water (Barrett
1995). The western portion, in Texas, is the focus of this study. It is
a "freshwater delta" (Barrett 1995) occurring on drowned
stream channels, point bar deposits, natural levees and lacustrine
deposits. These features provide for a diversity of communities
including ponds, cypress swamps, hardwood flats and mesic communities.

METHODS

Two transects, perpendicular to Cypress Bayou, were randomly
located along a 1.5 km baseline in the Caddo Lake State Park and
Wildlife Management Area. The transects, with a combined length of 1500
m followed a moisture and elevation gradient from a mesic natural levee (53.4 m above sea level) through a bottomland oak flat, to a cypress
swamp (50.6 m above sea level). Thirty points, each defining the center
of a series of nested plots, were located at 50 m intervals along the
transects. The samples, a modified and simplified form of the
"Whitaker method" (Shmida 1984), included a series of nested
plots. Each point defined a 1000 [m.sup.2] circular plot, a 500
[m.sup.2] circular plot, a 100 [m.sup.2] circular plot, a 3.16 m by 3.16
m (10 [m.sup.2]) plot, and a 1 [m.sup.2] plot. Plot centers were
permanently marked with tagged aluminum stakes. Inundated plots were
marked by recording the distance and azimuth from a nearby tagged
reference tree.

Diameter at 1.5 m (breast) height (dbh) was recorded for each tree
greater than 10 cm dbh in the 500 [m.sup.2] plot and the number of stems
of shrubs and saplings less than 10 cm dbh but greater than 1 m high was
tallied for species in the 100 [m.sup.2] plot. Ground layer species
(herbaceous species and woody plants less than 1 m tall) were assigned
"occurrence ranks" based on their occurrence in different
sized plots. Species occurring in 1 [m.sup.2] plots were assigned an
occurrence rank of "five" while those found in the 10
[m.sup.2] plot but not in the 1 [m.sup.2] plot received a rank of
"four", and those occurring in the 100 [m.sup.2] but not the
smaller plots received a rank of "three". Species not found in
the 100 [m.sup.2] plot but observed within a radius of approximately 18m
(1000 [m.sup.2] area) of the plot center were given a rank of
"two" if three or more individuals or colonies were
encountered and a rank of "one" if only one or two individuals
were encountered or if plants were found only in one or two small,
localized colonies. Percentage cover was also estimated for all ground
cover species in the 10 [m.sup.2] plot. Species not found in the 10
[m.sup.2] plot but in the larger sample area were given a
"trace" coverage of 0.1%. The coverage, projected on the
ground, and occurrence of the epiphyte Tillansia usneoides (L.) L. was
included with the ground layer.

The depth of water covering inundated plots or the depth to water
in a shallow pit on non-flooded plots was measured for each plot on 6
December 1996, a time when the majority of the plots were inundated and
the water level at a nearby gauging station (Tall Pines Lodge) was
observed at 52.03 m above mean sea level. A level and sighting-stick
were used to measure the elevation of two plots that were more than 1m
above the water. Since the gauging station was relatively close (<
1000 m) to the plots and negligible current was observed, variation in
the elevation of the water surface or "head" was considered to
be negligible. The elevation of each plot was estimated by subtracting
the depth of the plot below water from the water level elevation (52.03
m).

Since a goal of this study was to survey and map the plant
communities of the entire Caddo Lake State Park and Wildlife Management
Area, a survey methodology was devised that would facilitate a rapid
survey of the area, but still obtain data suitable for quantitative
analysis. Polygons, each representing a distinct patch of vegetation,
were drawn on January 1993 color infrared aerial photographs covering
the Caddo Lake State Park and Wildlife Management Area. During the
summer and fall of 1994 and 1995, 119 of these areas were visited and a
representative portion of each area roughly 1 ha in size was surveyed.
All overstory species (> 10 cm diameter at 1.5 m (breast) height
(dbh), mid story species (< 10 cm dbh but > 1 m high), and ground
layer (herbaceous plants and woody plants < 1m high) were identified
and ranked on a five-point abundance scale within their respective
stratum. Notes on hydrology, soils, and wildlife also were recorded. In
addition to providing a survey of most of the Park, these data provided
an independent data set with which to corroborate the results of the
transect plot data.

Voucher specimens were deposited in the herbarium of Stephen F.
Austin State University (ASTC). Nomenclature follows Hatch et al. (1990)
and Kartesz (1994) for species not found in Hatch et al. (1990).

Data analysis. -- Databases were compiled for both the transect and
survey data sets. Overstory and ground layer data from both data sets
were separately subjected to Detrended Correspondence Analysis (DCA)
ordination (Hill 1979a; Hill & Gauch 1980) and Two Way Indicator
Species Analysis (TWINSPAN, Hill 1979b): TWINSPAN is a hierarchical,
divisive polythetic classification which classifies samples into groups
with similar species composition. Input data for each species in each
sample included log-transformed density (recommended by Harcombe et al.
1993) for the transect overstory, occurrence ranks for the transect
ground layer, and abundance ranks for the survey data. Linear regression (Steele & Torrie 1980; Ludwig & Reynolds 1988) was used to
relate DCA scores to water levels for the transect data set. Field notes
were used to relate a DCA of the 119 survey sites to a flooding gradient
and to other observed environmental factors.

[FIGURE 1 OMITTED]

DCA and other ordination techniques summarize complex,
multivariate, samples-by-species data sets by arranging samples
objectively along several axes on the basis of their species
composition. Vegetation samples can be graphed as a scatter diagram based on ordination scores, in which points that are near each other
represent samples with similar species composition while points distant
from each other represent samples with dissimilar species composition
(Hill & Gauch 1980; Gauch 1982; Jongman et al. 1995). Ordination
axes are generally interpreted as being gradients in species composition
reflecting an underlying environmental gradient such as one of flooding
or nutrients, (Gauch 1982; Ludwig & Reynolds 1988).

[FIGURE 2 OMITTED]

TWINSPAN and DCA results for both the overstory and ground layer,
field notes and observations of aerial photographs were used to classify
the 119 survey samples into community types. Since plant communities
generally change continuously along environmental gradients, (Gleason
1926; Gauch 1982) all classifications are somewhat artificial; arbitrary
decisions occasionally had to be made among adjacent types when there
was lack of agreement between results for overstory, ground flora,
TWINSPAN and DCA. Ordination results were usually given preference over
TWINSPAN results for these decisions. The classification was
corroborated by comparison of the community descriptions with the
results from the transect data set and was used along with aerial
photographs to develop a map of the wetland vegetation of Caddo Lake
State Park and Wildlife Management Area and adjacent areas.

[FIGURE 3 OMITTED]

DCA ordinates species simultaneously with samples (Hill & Gauch
1980; Hill 1979a). Species occurring nearby on an ordination tend to
occur in the same type of samples, while distant species are generally
found in different types of samples. Likewise TWINSPAN classifies
species into groups on the basis of the sites they occur in (Hill 1979b;
Gauch 1982). TWINSPAN and DCA results were used to classify Caddo Lake
species into "ecological species groups": groups of species
which respond similarly to environmental gradients and tend to occur
together on similar types of sites (Muller-Dombois & Ellenberg 1974;
Barnes et al. 1982). Presence of several members of a group is a strong
indication of a site's ecological conditions and community type.
For ground layer species, only those listed as community indicators or
strong preferentials (occurred in at least 57% of the samples for the
group) during one of the divisions of TWINSPAN were included in the
ecological species groups.

RESULTS

Gradient analysis. -- The first axis of a DCA ordination of the 30
transect plots based on the occurrence rank of ground layer species
(Figure 1) was strongly correlated with water levels on the plots
([R.sup.2] = 0.74, p < 0.01, Figure 2). The ordination reflected a
gradient from well-drained mesic sites on a natural levee along Cypress
Bayou (high first DCA axis ordination scores) to seasonally flooded
oak-dominated flats (intermediate scores) to semi-permanently inundated
water elm and cypress swamps (low scores). TWINSPAN classified the plots
into five groups (the symbols in Figures 1 and 2) and showed close
agreement with the DCA arrangement (Figure 1).

A DCA ordination of the 30 transect plots on the basis of
log-transformed density of overstory trees provided an arrangement of
samples much like that of the ground layer ordination (Figure 3). As
with the ground layer, first axis DCA scores were strongly correlated
with water levels ([R.sup.2] = 0.84, p < 0.01, Figure 2). Wet
Bottomland Oak Flats with their overcup-oak dominated canopy were more
distant from the other bottomland hardwood samples than for the ground
layer ordination. The overstory ordination arrangement also corresponded
closely with the ground-layer TWINSPAN classification, represented by
the symbols displayed in Figures 1, 2 and 3).

Mean overstory density (Table 1) for mesic and Bottomland Oak Flat
communities was generally in the range of the 300-600 stems/ha that was
reported for the Big thicket (Marks & Harcombe 1981). Maximum
density was observed among the Cypress-Water elm Swamps, while
Open-canopied Cypress Swamps in deeper water showed the lowest density
(Table 1). Excluding the open Erianthus sites, mean basal area for the
bottomland Oak Flats (28 [m.sup.2]/ha, Table 1) was similar to the 29
[m.sup.2]/ha reported on equivalent sites from the Big Thicket (Marks
& Harcombe 1981), but was lower than that reported from other
southeastern bottomland forests (Robertson et al. 1978). Basal area was
higher for the swamps (Table 1), in part because of buttressing of the
stems. However, only one Closed-canopy Cypress Swamp sample (90
[m.sup.2]/ha) approached the 130 [m.sup.2]/ha reported for the
cypress/tupelo stand described in Marks & Harcombe (1981).

[FIGURE 5 OMITTED]

The first axis of a DCA ordination of the 119 survey areas based on
ground layer species arranged samples in a pattern similar to that of
the transect data set (Figure 4). As with the transect data, the
ordering of sites reflected a gradient from semi-permanently flooded
swamps to seasonally-flooded oak flats to temporarily-flooded natural
levees and terraces. However, it was not possible to identify subgroups
of Bottomland Oak Flats with this less precise and coarser-scale data
set. The first axis of a DCA ordination based of the abundance ranks of
overstory trees from the 119 survey sites was highly correlated
([r.sup.2] = 0.95, p < 0.01) with the first axis of the survey ground
layer ordination and reflected a gradient similar to that of the ground
layer. The second axis of the ground layer ordination was largely
related to variation among mesic sites (Figure 4). Sites with high axis
2 scores were associated with steep slopes and narrow ravines near the
Caddo Lake State Park campground and headquarters area while terraces
and natural levees from higher portions of the Cypress Bayou floodplain
had low scores. Ravines and slopes did not occur on the transects which
were wholly within the floodplain.

Ecological species groups. -- DCA orders species simultaneously
with samples, displaying near one another in ordination space those
species that occur in similar sites and are presumably ecologically
similar (Hill & Gauch 1980). Since species ordinations are based on
same information that samples ordinations are (Gauch 1982), the ordering
of the 136 ground layer species encountered in the 30 transect plots
along the first DCA axis was strongly related to water depth as was the
first axis of the samples ordination. Species characteristic of well
drained mesic sites (Chasmanthium sessiliflorum (Poir) Yates) had low
first axis scores, species found on seasonally flooded oak flats (Carex
joorii) had intermediate scores, and swamp species (Ceratophyllum
demersum L.) had high scores (Figure 5). TWINSPAN classified ground
layer species into five ecological species groups; groups of species
that respond similarly to environmental factors and tend to occur
together on similar types of sites (Mueller-Dombois & Ellenberg
1974; Barnes et al. 1982). The species groups are listed in Table 2 and
their species plotted in ordination space in Figure 5. Only 44 species
listed as indicator species or observed to be strong preferential
species (present on more than 57% of the samples for a given TWINSPAN
division) were included in the species group list. Twenty-eight of the
136 species encountered had a mean coverage of [greater than or equal
to]0.75% for at least one community type (Table 3). Most of these
dominant species also had indicator value as a comparison of Tables 2
and 3 reveals. Species groups followed the flooding gradient from the
mesic Chasmanthium group to the aquatic Ceratophyllum group (Table 1,
Figure 5). Although 363 species were encountered in the much larger
survey data set, the classification and list of indicator species from
the 119 survey areas was similar to that from the plots. Overstory
species for the transect plots were likewise classified by TWINSPAN
(Table 1).

[FIGURE 6 OMITTED]

It is also informative to observe changes in abundance for
individual species along an environmental gradient (Figure 6). Generally
the distributions species along the flooding gradient in this study
(expressed by the first DCA axis) are consistent with the bell-shaped
Gausian curves characteristic of the distributions of many species along
an environmental gradient (Gauch 1982). For example, cypress (Taxodium
distichum) seedlings plotted along the first ground layer DCA axis for
the survey data reach their peak abundance in shallow cypress-water elm
swamps and decline on drier sites and in deeper water where germination is limited by lack of soil exposure (Figure 6). Likewise, Ceratophyllum
demersum is abundant on swamp sites but becomes progressively rarer on
drier sites. Carex joorii shows peak abundance near the middle of the
gradient among the bottomland oak flats, while Chasmanthium
sessiliflorum is most abundant on mesic sites. Distributions for other
species can be inferred from observing rows in Tables 1-3.

Community types. -- The 119 survey samples were classified into six
community types (Rich-Mesic Slopes and Creek Bottoms, Mesic Bottomland
Ridges, Bottomland Oak Flats, Cypress Water-elm Swamps, Closed Canopy
Cypress Swamps and Deep-Water Open Cypress Swamps) on the basis of
multivariate analysis of both overstory and ground flora (Figure 4).
This classification forms the basis for a map of the wetland vegetation
of Caddo Lake State Park and Wildlife Management Area (Figure 7).
Fifty-two percent of the samples (Figure 4) and 56% of the area mapped
in Figure 7 consisted of one of the three Taxodium dominated swamp types
(Table 4); a much higher proportion of cypress swamps than that of any
other Texas wetland landscape the authors observed.

(1) Rich Mesic Slopes and Creek Bottoms: These hardwood-dominated
communities occurred on steep sheltered slopes, creek bottoms, and
ravines along the edge of the Caddo Lake basin. The community was mainly
observed in the campground and headquarters portion of Caddo Lake State
Park, which according to USGS topographic maps, has some of the greatest
topographic relief on the Caddo watershed. Ground flora included members
of the Chasmanthium group as well as a rich assemblage of mesophytic
plants including Florida maple (Acer barbatum Michx.), cross vine
(Bignonia capreolata L.), Christmas fern (Polystichum acrostichoides
(Michx.) Shott.) and Canada snakeroot (Sanicula canadensis L.) that were
not found elsewhere. Unlike the remaining five types, these are not
wetlands and they did not occur within the Caddo Lake floodplain
(<54m elevation). However, these sites have better moisture relations
than the adjacent uplands and were possibly better protected from
presettlement fires by their topographic location.

[FIGURE 7 OMITTED]

(2) Mesic Bottomland Ridges and Flats: These rarely-flooded
communities, found within the Cypress Bayou floodplain (<54 m
elevation), occurred on the crests of natural levees and meander scrolls
that developed along stream channels and along the gradually sloping
Pleistocene low terraces adjacent to the lower wetlands. Stands were
commonly dominated by loblolly pine (Pinus taeda L.), sweetgum
(Liquidambar styraciflua L.), southern red oak (Quercus falcata Michx.)
and other hardwoods. Important shrubs included Ilex decidua Walt and
Vaccinium arboreum Marsh. Ground vegetation was dominated by members of
the Chasmanthium and Carex joorii groups especially Chasmanthium
sessilflorum, sedge (Carex joorii) and greenbriar (Smilax spp.).

(3) Bottomland Oak Flats: These communities occurred on the lower
portions of islands and levees that gently slope down into the wetter
swamps. Sites were seasonally flooded and soils poorly drained, but were
above water for most of the year. Most stands were dominated by willow
oak (Q. phellos), with lesser amounts of overcup oak (Q. lyrata),
blackgum (Nyssa sylvatica Marsh.) and sweetgum (Liquidambar
styraciflua). Wetter stands transitional to swamps were generally
dominated by Q. lyrata. Important shrubs included Crataegus opaca Hook
& Arn., Diospyros virginiana L., Styrax americana Lam., Ilex decidua
and on lower sites, Foresteria acuminata (Michx.) Poir. Ground
vegetation included Carex joorii, and other members of the C. joorii
group. Members of the Brunnichia group were also common, especially on
wetter sites. Bottomland Oak Flats can be divided into three community
subtypes: (a) willow oak flats, dominated by willow oak and C. Joorii,
(b) sites on slight mounds with a somewhat open canopy of willow oak and
a dense ground cover of Erianthus strictus and (c) overcup oak-dominated
flats on lower, wetter sites. The environmental and/or historic factors
maintaining the canopy openings of the Erianthus subtype are not known,
but the groundflora appears to be transitional between Mesic Bottomland
Ridges and the remaining Bottomland Oak flats (Figure 1, Table 1).

(4) Cypress Water-elm Swamps: Cypress-water elm communities were
transitional between oak flats and swamps. Occurring slightly below the
current normal pool elevation of the lake (Figure 2), they were flooded
for much of the year, but had significant periods of exposure. Cypress
dominated the overstory but there was an abundant sub-canopy of water
elm (Planera aquatica). Shrubs were sparse, but included Foresteria
acuminata, and Cephalanthus occidentalis L. The Ground layer was sparse
during low water periods, largely represented by the Brunnichia group.
When flooded, species from the Ceratophyllum group were found.

(5) Closed-Canopy Cypress Swamps: These swamps, often associated
with abandoned stream channels, occurred lower on the landscape than the
cypress water-elm swamps, and were usually under water. The high density
of cypress indicated that historically there were periods in which the
sites were exposed enabling significant regeneration. The tree layer was
pure baldcypress (T. distichum). Shrubs were limited to scattered
Cephalanthus occidentalis growing from stumps and logs. By mid summer,
the surface was covered by members of the Ceratophyllum group including
Egeria densa, fanwort (Cabomba caroliniana) duckmeat (Spirodela punctata
(Meyer) Thomps.), water meal (Wolffia columbiana Karst) and water fern (Azolla caroliniana Willd.).

(6) Open (Deep Water) Cypress Swamps: In deeper water, cypress
stands became less dense. The deepest swamps were reduced to scattered
trees growing in open water covered by floating and submersed aquatic
plants. As with the previous community, members of the Ceratophyllum
group dominated the surface. Dense colonies of Nuphar luteum were also
common.

DISCUSSION

Caddo Lake wetland vegetation corresponds to a flooding and
elevation gradient from mesic, rarely-flooded terraces and levees to
nearly continuously flooded cypress swamps. While removal of the Great
Raft and dam construction may have modified hydrology, modern vegetation
appears to be closely adjusted to the current flooding regime as
evidenced by the correspondence between the normal pool elevation of
51.36 m and the boundary between semi-permanently flooded swamps and
seasonally flooded Bottomland Oak Flats (Figure 2).

Other bottomland and swamp vegetation studies. -- Other authors
have described hydrologic gradients and plant communities comparable to
those at Caddo Lake from other locations in the South. Crouch &
Golden (1997) surveyed the flora of an Alabama bottomland forest where a
seasonally flooded floodplain was dominated by bottomland hardwoods
including Quercus phellos, Q. pagoda Raf., Q. nigra L and L.
styraciflua, with Q. lyrata and Carya aquatica (Michx f.) Nutt.
dominating a somewhat lower slough. An area of slopes and ravines
adjacent to the floodplain contained a rich mixture of hardwoods similar
to the Rich Mesic Steep Slopes of Caddo Lake.

Thompson (1980) described mesic terrace bottoms, mixed softwood
levees, oak hardwood bottoms and shallow Taxodium distichum swamps from
a floodplain forest in Missouri. The recently-formed mixed softwood
levees, characterized by pioneer trees such as Salix nigra Marsh. were
not observed at Caddo Lake, possibly because the high-energy stream
flows responsible for creating such landforms have not been present on
Big Cypress Bayou during the last 200 yr as a result of its partial
impoundment. Although many southern bottomland species were present,
Thompson (1980) also reported a number of species such as Acer
saccharinum L. and Quercus palustris Muenchh. from this more northerly
site that were not found at Caddo Lake.

Texas and Louisiana studies. -- Marks & Harcombe (1981)
identified Floodplain Hardwood Forest and Swamp Cypress-Tupelo Forest
from the Big Thicket of southeastern Texas. Their two highest Floodplain
Hardwood stands are roughly equivalent to Mesic Bottomland Ridges,
except that Fagus grandifolia, Ehrh., abundant in the Big Thicket sites,
was absent at Caddo. The remaining Big Thicket Floodplain Hardwood
stands correspond to Caddo Lakes's Bottomland Oak Flats except that
Quercus lyrata is less important for the Big Thicket and Carpinus
caroliniana Walt., abundant in the Big Thicket understory, is absent at
Caddo. Nyssa aquatica L., dominant in the Big Thicket swamp, was absent
from the corresponding swamps at Caddo Lake.

Mathies et al (1983) described a series of wetland communities from
Cunningham Brake in Nachitoches Parish, Louisiana. Taxodium distichum
and N. aquatica, were associated with areas flooded for most of the year
and lowland hardwoods including Q. phellos, Q. nigra, Q. laurifolia
Michx., Q. falcata (=Q. pagoda ?), and L. styraciflua dominated
seasonally flooded bottomlands. Open sand bars and stream edges along
Kisatchie Bayou, characterized by pioneer species such as Betula nigra
L. and Salix nigra, are equivalent to the softwood levees of Thompson
(1980), but with the exception of a few sites observed on the spoils of
artificial ditches, were not observed at Caddo Lake.

Relation to regional classification systems. -- Van Lear &
Jones (1987) developed an ecological site classification system for
terraces and floodplains associated with the Savannah River in South
Carolina. Resembling the Caddo Lake communities, ecological land units
are arranged along a flooding gradient from deep water cypress swamps,
shallow cypress and tolerant hardwood swamps, laurel oak-water oak
floodplains characterized by prolonged flooding, and loblolly pine-sweetgum-oak communities on terraces above the active floodplain.

Larson et al. (1981) and Christenson (1988) described a "zonal
classification" of southeastern alluvial wetlands which divides
them into five zones based on the frequency and duration of flooding.
Mesic Bottomland Ridges essentially correspond to zone V (infrequently
flooded transition to uplands), Bottomland Oak Flats correspond to zone
IV (seasonally flooded forests of backwaters and flats), Overcup
oak-dominated lower Bottomland Oak Flats correspond to Zone III (Lower
hardwood swamp forest), and the three swamp types described for Caddo
lake correspond to zone II (intermittently exposed river swamp forest).
Zone I represents the permanent water of river channels and lakes.

In 1998, The Nature Conservancy released a draft of a comprehensive
classification of terrestrial vegetation for the southeastern United
States. The system classifies vegetation into Alliances which are in
turn made up of one or more Associations (Weakley et al. 1998). While
the Caddo Lake communities can generally be placed into an appropriate
Alliance, in many cases an Association has yet to be described that
closely fits them. Additional quantitative case studies such as this one
will be invaluable in refining the classification and in increasing our
understanding of regional patterns of vegetation.

Mesic bottomland Ridges appear to best fit in the Pinus
taeda-Quercus (phellos/nigra/laurifolia) Temporarily Flooded Forest
Alliance (I.C.3.N.b.070), but they also have affinity to the Pinus
taeda-Quercus (pagoda/shumardii/michauxii) Temporarily Flooded Forest
Alliance (I.C.3.N.b.060). The willow oak-dominated Bottomland Oak Flats
best fit into the Quercus phellos Seasonally Flooded Alliance
(I.B.2.N.e.130), but the Alliance usually occurs in upland depressions,
rather than bottomlands. Willow Oak Flats also resemble the Quercus
(phellos/nigra/laurifolia) Temporarily Flooded Forest Alliance
(I.B.2.N.d.250) although there are understory differences and Big
Cypress Bayou is not a "blackwater or low sediment/nutrient
river" as is associated with this Alliance (Weakley et al. 1998).
The lower, overcup oak-dominated Bottomland Oak Flats belong to the
Quercus lyrata (Carya aquatica) Seasonally Flooded Forest Alliance
(I.B.2.N.e.100) and possibly to the Quercus lyrata/Liquidambar
styraciflua/Foresteria acuminata Forest Association. Cypress-Water elm
Swamps best fit the Taxodium distichum/Nyssa Seasonally Flooded Forest
Alliance (I.B.2.N.e.190) although Nyssa spp. were absent from Caddo
swamps. Weakley et al. (1998) also described a Planera aquatica
Seasonally Flooded Forest Alliance (I.B.2.N.e.090) with an understory
similar to that observed in Cypress-Water elm Swamps at Caddo Lake.
However, the emergent cypress canopy at Caddo is too dense to fit the
description for this Alliance. Closed Canopy and Open Cypress Swamps
belong to the Taxodium distichum Semi-permanently Flooded Forest
Alliance (I.B.2.N.f.060) and to the Taxodium distichum/Lemna minor.
Association, although at Caddo Spirodela spp. and Wolfia spp. dominate
the surface rather than Lemna spp. Some of the larger openings in the
Open Cypress Swamps (such as portions of Clinton Lake, Figure 7) belong
to the Nuphar lutea Permanently Flooded Herbaceous Alliance
(V.C.2.N.a.040) where Nuphar dominates, or to the southern variant of
the Potomogeton spp./Ceratophyllum spp./Elodea spp. Permanently Flooded
Herbaceous Alliance (V.C.2.N.a.065).

The authors acknowledge the National Biological Service for their
support of this study. We also thank Jim Neal, Jim Johnston, William
Sheffield, Tommy Pritchard, Edward Hughs and Virginia Burkett for their
contributions. Dr. P. A. Harcombe and three anonymous reviewers provided
valuable comments on the manuscript.

Hill M. O. 1979b. TWINSPAN--a FORTRAN program for arranging
multivariate data in an ordered two-way table by classification of the
individuals and attributes. Ithaca, New York. Ecology and systematics,
Cornell University.